Bacillus anthracis is the etiologic agent responsible for anthrax, a disease often found in persons exposed to infected animals or their products. Persons particularly exposed to animals include veterinarians, laboratory technicians, ranchers and employees working with skin or hair of animals. The mode of entry into the body may be the skin or, when contaminated meat is eaten, the gastrointestinal tract. Inhaling of spores can cause inhalation anthrax, a disease that can be fatal. Vaccines against Bacillus anthracis have been available. Virulent strains of the organism produce two toxins and a poly-D-glutamic acid capsule which are coded for on two endogenous plasmids, pX01 and pX02, respectively. Loss of either of the plasmids results in an attenuated strain of reduced virulence, while loss of both results in an avirulent organism. The history of the USAMRIID Sterne strain of B. anthracis prior to 1981 is uncertain, though it is believed to be derived from the Sterne strain isolated at the Onderstpoort Research Laboratory in Pretoria, South Africa.
In 1985 the Bacillus anthracis protective antigen (PA) gene was cloned into a plasmid (pUB110) resulting in the formation of a recombinant plasmid identified as pPA102, which was reported in the literature (Ivins and Welkos, Infection and Immunity, S54:537-542 (1986)). The production of vaccines lacking lethal factor was possible thereby. However, a primary problem remained, since the Bacillus anthracis formed spores. Once spores have formed, they persist in the environment for months and years. Once the laboratory environment contains such spores, it is very difficult to free the environment of the spores.
It was also previously reported that protective antigen (PA) could be produced in baculovirus. [Iacono-Connors, et al., Infection and Immunity, 58:366-372 (1990); Iacono-Connors, et al., Infection and Immunity, 59:1961-1965 (1991)] A major problem in production of the PA in the baculovirus disclosed therein is that the desired antigen requires a complex purification process. Even after purification by immuno-affinity chromatography, undesired cellular material continues to contaminate the desired product.